Definition: optical modulators based on the Franz–Keldysh effect
More general terms: optical modulators
An electroabsorption modulator (or electro-absorption modulator) is a semiconductor-based optical modulator. It can be used for controlling (modulating) the intensity (more precisely: the optical power) of a laser beam via an electric voltage (→ intensity modulators). Its principle of operation is based on the Franz–Keldysh effect [1, 2], i.e., a change in the absorption spectrum caused by an applied electric field, which changes the band gap energy (thus the photon energy corresponding to an absorption edge) but usually does not involve a substantial excitation of carriers by the electric field.
Most electroabsorption modulators are made in the form of a waveguide with electrodes for applying an electric field in a direction perpendicular to the modulated light beam. For achieving a high extinction ratio, one usually exploits the quantum-confined Stark effect in a quantum well structure.
Compared with electro-optic modulators, electroabsorption modulators can operate with much lower voltages (a few volts instead of hundreds of thousands of volts). They can be operated at very high speed; a modulation bandwidth of tens of gigahertz can be achieved, which makes these devices useful for optical fiber communications. A convenient feature is that an electroabsorption modulator can be integrated with a distributed feedback laser diode on a single chip to form a data transmitter in the form of a photonic integrated circuit. Compared with direct modulation of the laser diode, a higher bandwidth and reduced chirp can be obtained.
Bibliography
[1]
L. V. Keldysh, “Behaviour of non-metallic crystals in strong electric fields”, J. Exp. Theor. Phys. (USSR) 33, 994 (1957); translation: Sov. Phys. JETP 6, 763 (1958)
[2]
W. Franz, “Einfluß eines elektrischen Feldes auf eine optische Absorptionskante”, Z. Naturforsch., Teil A 13, 484 (1958)
[3]
E. Lach et al., “Application of electroabsorption modulators for high-speed transmission systems”, J. Opt. Fiber Commun. Rep. 2, 140–170 (2005)
[4]
J. Liu et al., “Waveguide-integrated, ultralow-energy GeSi electro-absorption modulators”, Nature Photon. 2, 433 (2008), doi:10.1038/nphoton.2008.99
